Toner particles for electrophotographic copying and processes for their preparation

ABSTRACT

Toner particles for electrophotographic copying and electrostatic printing consist of pigmented thermoplastic base particles having the surface covered with a thermoplastic fine-grained polymerizate. The base particles are prepared by suspension polymerization and the fine-grained polymerizate originates from a latex prepared by emulsion or microsuspension polymerization. One method of preparing the toner consists of bringing an aqueous dispersion of the base particles into contact with a latex of the fine-grained polymerizate. The temperature is raised so that the fine-grained particles adhere to the surface of the base particles. A protective colloid system can be present and the particles can have charges of opposite character. In another method the preparation of the toner particles comprises suspension polymerization of monomer for formation of the base particles in the presence of an already prepared latex whereby the latex particles have higher hydrophilicity than the polymer in the base particle. The fine-grained particles can also be applied to the base particles according to a dry method.

The present invention relates to toner particles for use inelectrophotograhic copying or electrostatic printing. More particularlythe invention relates to such toner particles having a pimply surface.The invention also relates to methods for the preparation of such tonerparticles.

In electrophotographic copying the latent print on the photo-drum isdeveloped with a toner consisting of fine-grained pigmentedthermoplastic particles. The most common method of preparing a tonercomprises melting a thermoplastic material and mixing this with pigment,charge modifiers, release agents etc. The product is then cooled,crushed, ground and screened in an air stream to obtain particles with asize in the order of from 5 to 30 μm. According to this method particlesof very varying shapes and sizes are obtained. This variation in shapeand size gives rise to certain disadvantages in the copying process.Efforts have thus been made to find processes which give toner materialshaving spherical and fairly uniformly sized particles.

One way of preparing a toner is to finely divide molten waxes or lowmolecular thermoplastic materials in a spray drier. If such conditionsare used at the spray drying that a suitable particle size is obtaineddirectly the grinding step can hereby be eliminated. A disadvantage ofthe spray drying is, however that the size distribution of the powder isfairly wide. Further, it is not possible to satisfactorily spray dry amelt of the thermoplastic materials which are usually used at so-calledheat fixing by means of hot rolls or radiation. Spray dried powders aremore suitable as toners intended for cold fixing by means of pressing.Spray dried particles are advantageous in that they are spherical andthus not of such varying shapes as ground particles.

One method of preparing toner particles suitable for hot fixing, whichmethod in principle is very cheap, is to disperse pigments, chargemodifiers, release agents, initiators etc in monomers which givepolymers having suitable properties for heat fixing. The monomer isemulsified with a suitable colloid system in water, the temperature israised and by polymerization a fine, pigmented powder of spericalparticles is directly obtained and this powder can be dried, afterwashing, to give a powder suitable for use in coping. Since the methodis very simple several attempts have been made to prepare tonerparticles in this way. One process of this kind is disclosed in theBritish patent application 2091435. In practical tests it has been foundthat particles prepared by suspension polymerization have certainadvantages such as the capability of giving copies with good resolution.However, it has also been found that such particles have a seriousdisadvantage in that they have such a strong adhesion to the photo-drumthat they are only very incompletely transferred to the paper. Suchparticles also agglomerate strongly with each other which means thatsuch a toner has very poor free-flowing properties etc.

The drawbacks with the particles mentioned above can be related to thesmooth surface of the spherical particles prepared by normal suspensionpolymerization. When spherical particles with a smooth surface are usedthe adhesion to the photo-drum will be so strong that they are not evencompletely removed at the cleaning of the photo-drum which follows thetransfer step. Such remaining toner thus leads to a rapid deteriorationof the quality of the copies. A strong adhesion to the photo-drum isparticularly serious if the toner consists of or contains very fineparticles, <5 μm, since it is more difficult to remove such fineparticles from the surface of the photo-drum by means of mechanicalcleaning systems.

According to the present invention it has been found that sphericalparticles having a pimply surface do not show the disadvantage of astrong adhesion to the photo-drum. In several cases such particles evenhave lower adhesion than toner particles prepared by the conventionalgrinding process.

The present invention thus relates to a toner which comprises aninternally pigmented base particle, or main particle, prepared bysuspension polymerization and having a mean diameter of from 2 to 25 μm,the surface of the particle being covered by a fine-grained polymerizatewith a mean diameter of from 0.05 to 33% of the mean diameter of thebase or main particle.

Internally pigmented base particles here refer to particles prepared ina manner where the pigment is mixed with the monomer before thepolymerization and thus is present more or less uniformly distributed inthe finished polymerized base particle.

The application of the fine-grained particles on the surface of thetoner particles can be carried out in a manner which is technically sosimple that the whole process from monomers to coated toner particles iseconomically advantageous. The invention therefore represents a moreeconomical way to make high quality toners.

The size of the fine-grained particles which form protuberances on thesurface of the base or main particles should be substantially smallerthan these particles. The diameter of the fine-grained particles shouldthus be maximum 33%, and preferably maximum 15%, of the diameter of thebase particle. The lower size limit is set by the smallest size whichgives the desired effect of reduced adhesion to the photo-drum. Alreadypimples having a size in the order of 0.005 μm give a reduced adhesion.The fine-grained polymerizate suitably has a particle size in the rangeof from 0.005 to 5 μm, preferably from 0.02 to 2 μm.

Another important factor is the degree of covering, ie how great a partof the surface of the particle which is covered by fine-grainedparticles. The closest possible covering corresponds to about 91% of thesurface of the base or main particle. However, such a high degree ofcovering is not necessary to reduce the adhesion to the photo-drum. Ithas surprisingly been found that already a degree of covering of from 1up to 10% gives a very strong reduction of the adhesion.

Toner particles according to the invention with fine particles formingprotuberances on the surface of the base or main particles can beprepared in a number of different ways as described below.

First will be described a process according to which fine-grainedparticles are adhered to the surface of base particles already preparedby suspension polymerization. To make the base particles monomer solubleinitiator, pigment and optional charge modifier and a dispersing agentfor the pigment are first mixed. The mixture is emulsified in waterusing a suitable colloid system. After evacuation the temperature israised for polymerization and spherical base particles are obtained. Themean diameter of these can be from 2 to 25 μm, preferably from 3 to 15μm.

The small polymer particles which are applied to the surface of the baseparticles can be prepared by emulsion polymerization or microsuspensionpolymerization in per se known manners and, if desired, charge modifiersand pigments can for example be incorporated at microsuspensionpolymerization.

It is desirable that the fine-grained particles are strongly anchored inthe surface of the base particles. This can be achieved by softening thebase particles using small amounts of softening agents or by heating.Hereby the fine-grained particles will be anchored by melting into thesurface. It is suitable that the fine-grained particles melt into thebase particles to a depth corresponding to about half the diameter ofthe fine-grained particle. However the depth can be varried, it is onlynessecary that the fine-grained particles are firmly anchored in thebase particles and at the same time protrude out from the surface ofthem.

The polymer composition of the fine-grained particles may be the same asthat of the base particle. However, it might be advantageous to choose amore high-melting polymer type for the fine-grained particles on thesurface. A higher melting point gives a smaller risk that thefine-grained particles will agglomerate with each other instead ofadhering to the surface of the base particles at the coating process. Inorder to make the fine-grained particles on the surface particularlydifficult to melt they can be cross-linked to a higher degree than thebase particles.

The fine-grained particles can be applied to the surface of alreadyformed base particles in wet or dry methods or according to a specialtechnique, described below, to the surface of the monomer droplets,which after polymerization together with the latex particles will formpimply particles.

A toner can for example be prepared by bringing an aqueous dispersion ofthe pigmented base particle into contact with a latex of thefine-grained polymerizate whereby a protective colloid system issubsequently formed in the aqueous dispersion and the temperature raisedto make the fine-grained particles adhere to the surface of the baseparticles.

At production according to this method the protective colloid systemwhich has been used for the suspension polymerization of the baseparticles is suitably first deactivated. If, for example, certaininorganic powder stabilisers have been used as protective colloids suchas difficultly soluble phosphates these can be dissolved by acidifyingthe aqueous suspension of the base particles. The latex of thefine-grained particles is then slowly added. Hereby such conditionsshould be used that the latex will not precipitate immediately oncontact with the suspension of the base particles, since there is then arisk that the small particles in the latex will agglomerate with eachother instead of being deposited on the surface of the base particles.

After agitation for some time the fine-grained particles will have beenprecipitated on the surface of the base particles. The system is thenmade more alkaline so that the protective colloid system will bereformed. There is then no risk of agglomeration of the particles whenthe dispersion is warmed to melt the fine-grained particles into thesurface of the base particles. Acidification and washing is then carriedout.

In some cases the protective colloid system can be kept intact when thelatex of the fine-grained particles is added. In this case latexparticles and base particles which have opposite charges are used. Thisis achieved through controlling the Zeta-potential of the particles. Thechemical composition of the surfaces of the base particles and the latexparticles are chosen in such a way that the two types of particles haveZeta-potentials of opposite character at the used condition. This can beaccomplished by copolymerization with functional monomers with chargesof opposite character. The charged latex particles will be attracted tothe base particles of opposite charge and thereby penetrate the colloidlayer.

To give the toner the correct triboelectric charge the fine-grainedparticles on the surface must have a specific triboelectric charge andthis can be achieved by subsequently precipitating a charge modifyingagent on the surface of the coated particles. If the fine-grainedparticles are prepared by microsuspension polymerization a chargemodifying agent can alternatively be mixed with the monomer alreadybefore the polymerization of the fine-grained particles. Finally thechemical composition of the fine-grained particles can be selected insuch a manner that no extra addition of charge modifying agent isrequired. Examples of such particles which give a positive triboelectriccharge are fine-grained particles of polyacrylonitrile oramino-containing monomer. When fine-grained particles of polyvinylchloride, fluoro polymers etc are used a negative triboelectric chargeis obtained.

The fine-grained particles can also be applied according to a drymethod. At application of the fine-grained particles by a dry method thebase particles are first dried and charged to a mixer. To keep a uniformmixture in the powder bed it is advantageous to admix larger beads, egglass beads of 5 mm. The small polymer particles which are to cover thesurface are then charged to the powder bed. The small particles can bepresent either as a dispersion in a suitable liquid, whereby the liquidis evaporated from the powder bed, or as an already dried veryfine-grained powder. The temperature of the powder bed is raised undercontinued agitation. The small particles will then adhere to the surfaceof the base particles and, at higher temperatures, at least partly meltinto the surface of the base particle.

An alternative way of making the small polymer particles melt into thesurface of the base particles is to introduce the coated base particlesinto an air stream which for a short time is heated to a temperature offrom 150° to 400° C., depending on the dwell time in the hot zone.

According to the above described methods the base particles are firstprepared by suspension polymerization and these are then treated with alatex of the fine-grained polymerizate to give "pimply" toner particlesaccording to the invention.

According to another method for the preparation of toner particles witha rough surface a special kind of process is used and herein a latex, iethe fine-grained polymerizate, is first prepared and the base particles,are prepared by suspension polymerization in the presence of the alreadyprepared latex. In the following this method of preparation, which formspart of the present invention, will be described more in detail.

According to the last method spherical particles with a pimply surface,useful as toners in electrophotographic copying and electrostaticprinting, are produced by first preparing a latex, an aqueousdispersion, of fine-grained polymer particles. The latex can be preparedaccording to the emulsion polymerization technique using water solubleinitiators and suitable emulsifiers or according to the technique ofmicrosuspension polymerization whereby the monomer is first finelydivided in water, by means of intensive emulsifying and usingsurfactants, and then polymerized using initiators which usually aresoluble in the monomer. In certain cases water soluble initiators can,however, be used in microsuspension polymerization.

The latex particles shall be insoluble in the monomers and optionallyother solvents and for this purpose they are preferably cross-linked.Further, the surface of the latex particles shall have a fixedhydrophilic/hydrophobic character.

At the production of pimply particles according to the method latex ismixed with a monomer or a monomer mixture. Monomer soluble initiator,pigment, charge modifying agent, release agent etc can have been addedto the monomer in advance. The mixing conditions, with regard to pH etc,should be selected in such a manner that the latex particles leave theaqueous phase and migrate to the monomer phase or to themonomer-water-phase boundary. Additional water and a suitable colloidsystem is then charged. The monomer is emulsified to small drops and thetemperature is raised for polymerization. After polymerization afine-grained pigmented powder is obtained. At investigation with ascanning electron microscope it is found that the latex particles ofsuitable hydrophilic/hydrophobic balance have migrated to the surface ofthe polymer particles formed at the suspension polymerization. Hereby apimply surface is obtained on these particles.

By varying the hydrophilic/hydrophobic balance it is possible to controlthe displacing of the latex particles with regard to the surface of theformed base particles. If the latex particles have a very hydrophobiccharacter, for example if they have been prepared from pure styrene,divinyl benzene and with hydrogen peroxide as initiator, they will notat all penetrate the surface of the formed polymer particles. Such latexparticles can thus not be seen by studying the formed polymer particlesin a scanning microscope. On the other hand, if the latex particles aretoo hydrophilic they can after the polymerization be found in theaqueous phase.

Suitable hydrophilicity for the latex particles depend on thehydrophilicity of the main particles. The latex particles should have ahigher hydrophilicity than the polymer of the main particles. The upperlimit for the hydrophilicity of the latex particles is the level wherethe latex particles start being pressed out from the main particles tothe aqueous phase during the polymerization.

The degree of hydrophilicity can for example be controlled at thepreparation of the latex particles by adding certain amounts of monomerwith anionic character in alkaline environment, for example methacrylicacid, itaconic acid, styrene sulphonic acid, etc. Compounds withcationic character in acid environment can also be incorporated in thelatex polymer to make the latex particles more hydrophilic, egtrimethylammoniumethyl methacrylate halide. However, it is not necessaryto use ionised groups to achieve hydrophilicity. Control ofhydrophilicity can also be achieved by polar, non-ionised monomers, egmethyl methacrylate, acrylo nitrile, allyl alcohol, 2-dimethylaminoethylmethacrylate, hydroxyethyl methacrylate. As polar nonionised monomerssuch containing amino groups or hydroxyl groups are preferred. It isalso possible to use amphoteric latex which contains both acid and basicgroups for obtaining a suitable hydrophilicity. The determining factorsfor the latex particles to form pimples according to this method arethus that the latex particles are not soluble in the monomer or themonomers which form the main particle and that the surface of the latexparticles have a more hydrophilic character than the polymer of theformed main particle.

Cross-linking is of course not necessary if the polymer composition ofthe latex particles is such that the latex particles are not soluble inthe monomers without being cross-linked. An example of this is latexparticles of polyacrylonitrile, or of copolymerizates having a highacrylonitrile content. Otherwise the latex particles are cross-linked toobtain insolubility.

The degree of cross-linking in the latex particles is of certainimportance. At a low degree of crosslinking the latex particles willswell in the monomers. The size of the pimples in the finished particleswill then be greater than the size of the particles in the used latex.Hereby a certain amount of latex will suffice to cover a greater part ofthe surface of the main particles. The swelling of the latex particlesalso results in that the difference in hydrophilicity between themonomer and the latex particle will be diminished since the compositionof the swelled latex particles will be more like that of the monomers.

The size of the pimples is, besides the degree of swelling, alsodetermined by the size of the particles in the latex. At a smaller sizea lower part by weight of latex is required to give a determined degreeof covering of the surface of the final particles. The smallest size ofthe pimples is decided both by the size the pimples should have tosufficiently eliminate the attraction to the photo-drum due to van derWaal forces or due to a too strong electrostatic adhesion and by thesmallest size which it is technically possible to prepare. With regardto van der Waal attraction this will decrease to a high extent alreadywhen the base particles, the main particles, are removed from each otherby 10 nm. If half the size of the pimples is outside the surface of thebase particle it should then suffice with a diameter of 20 nm for theparticles in the used latex. The largest size of the pimples is adiameter corresponding to about 33% of the diameter of the sphericalmain particle. The mean diameter of the particles in the latex should befrom 0.05 to 33% of the mean diameter of the main particle, and thisshould be within the range of from 2 to 25 μm.

The chemical composition of the latex particles forming the pimples canbe selected arbitrarily as long as the particles are not dissolved inthe monomer or monomer mixture used for formation of the main particle.As has been stated, the hydrophilic-hydrophobic balance must, however,be considered. Further, the fact that the pimples influence thetriboelectric properties of the final particle must also be considered.The pimples represent the outer contour of the toner particles. Atrubbing, the type and level of the triboelectric charge is thusdetermined by the chemical composition of the pimples. Further it shouldbe considered that the electrostatic charge will be greater withprotuberances in the form of pimples due to the increased surface of thepowder particles. The pimples may also influence the speed incharging/decharging of the particles because their small radius ofcurvature compared to the radius of the base particle.

The degree of covering, is an important factor and we have found thatalready a covering of from 1 up to 10% has a positive effect.

The property of the pimply spherical particles to give a decreasedadhesion to surfaces and between the particles themselves can giveadvantages also in other fields than electrophotographic copying. It hasthus been found that they give a reduced adhesion to the screen if theyare used for electrostatic printing according to the "dry silk-screen"method. Such prints will thus have a stronger colouring than ifspherical particles with smooth surface are used.

The rough surface results, as has been mentioned, in a reduced mutualattraction between the particles. A powder of such particles will thushave better free-flowing properties. Owing to the low tendency toformation of powder aggregates particles with a rough surface areadvantageously used also in powder coating operations, for example forcoating of metal articles when the powder is sintered on.

Choice of material will be discussed more in detail below and whennothing else is stated this is valid independent of the method ofpreparation of the base or main particles coated with fine-grainedpolymerizate.

As monomers, alone or in mixture, for the preparation of the latexparticles the following can for example be used: styrene and differentderivatives of styrene, acrylic acid and methacrylic acid or estersthereof, acrylo nitrile, vinyl chloride, vinyl fluoride, vinylidenefluoride, vinyl acetate etc. To obtain cross-linking polyfunctionalmonomers can be used, eg divinyl benzene, ethylene glycol diacrylate,ethyleneglycol dimethacrylate, trimethylolpropane triacrylate etc. Theamount of cross-linker can be varied to a high degree as long as thelatex particles fulfil the requirements on correct hydrophilicity andinsolubility.

For the preparation of the base particle, or the main particle, the samemonomers, and also cross-linkers, as above can be used, but generallysuch a mixture is used that this particle will have a lower softeningpoint than the latex particles.

The preferred main monomers for both latex particles and base or mainparticles are styrene, acrylates and methacrylates.

As emulsifiers for the preparation of the latex particles conventionalsurfactants for emulsion and microsuspension polymerization respectivelyare used. However, care should be taken that the emulsifier system willnot to a too high degree negatively influence the function of thecolloid system which is used for the preparation of the base particle orthe main particle. It is also advantageous if the used emulsifiers havesuch a water solubility that they can be washed away from the surface ofthe produced pimply toner particles.

As initiator at the production of the latex particles according toemulsion polymerization technique conventional water soluble initiatorscan be used, eg persulphates, hydrogen peroxide, hydroperoxides, etc.For latex according to the microsuspension polymerization techniqueconventional monomer soluble initiators can be used, eg dialkylperoxidicarbonates, tert.butyl peroxipivalate, octanoyl peroxide,lauroyl peroxide, tert.butyl peroxy(2ethylhexanoate), benzoyl peroxide,2,2-azobisisobutyronitrile, 2,2-azobis-2,4-dimethylvaleronitrile andsimilar compounds. For the preparation of the base or main particles thesame initiators as those for preparation of latex according tomicrosuspension polymerization can be used.

As protective colloid in the colloid system for the preparation of baseor main particles, water soluble colloids of the type cellulosederivatives, polyvinyl alcohol etc or powder stabilisers of the typedifficultly soluble phosphates, metalhydroxides, silica etc can be used.The powder stabilisers are preferably used together with a suitableco-stabiliser.

As colorants to be mixed with the monomer for the base or main particlesinorganic colorants, organic colorants, magnetite or carbon black areused. In certain cases it is suitable to give the pigments a surfacetreatment so that they will remain finely divided in the monomer drops.Toner particles according to the invention will thus be coloredthroughout, ie the colorant is present included, and more or lessuniformly distributed in the polymerice base or main particle. As hasbeen stated it is also possible to let colorant and certain otheradditives be present in latex prepared according to microsuspensionpolymerization processes.

The toner particles of the invention can be used together withconventional carriers for developer composition in known manner. Theinvention is further illustrated in the following examples which,however, are not intended to limit the same. Parts and percent relate toparts by weight and percent by weight, unless otherwise stated.

Example 1-11 relate to the first described method with its differentvariations, where fine-grained particles are adhered to the surface ofbase particles.

Example 12-16 relate to the second described method with fine-grainedparticles present at the preparation of the base particle.

EXAMPLE 1

Preparation of a fine-grained polymerizate. Emulsion polymerization.

40 g of styrene, 1.8 g of sodium dodecylsulphate and water to totally395 g were charged to a 500 ml glass flask equipped with cooling means,agitator and a valve for evacuation and introduction of nitrogen. Themixture was heated to 80° C. under agitation. At 80° C. 5 g, 3.5% ofhydrogen peroxide was added and the same time the mixture was subjectedto a nitrogen atmosphere.

The polymerization was then allowed to continue for 12 hours which gavea 0.11 μm 10% seed latex.

120 g of the above seed latex, 200 g of 5 g/kg sodium dodecylsulphate,0.4 g of divinyl benzene, about 50%, 27.6 g of styrene and water tototally 395 g were charged to the same equipment as above and the sameprocedure was then followed. In this manner a cross-linked polystyrenelatex was prepared with particles of 0.16 μm and a dry content of 10%.

EXAMPLE 2

Preparation of a fine-grained polymerizate. Microsuspenionpolymerization.

250 g of styrene, 0.8 g of neozapon schwartz X 51 charge modifier (fromBASF) and 2.5 g of 2,2'-azobis (2,4-dimethylvaleronitrile) were chargedto the same equipment as in Example 1. The monomer mixture was heatedand the bulk polymerization allowed to continue for two hours at 85° C.which resulted in a viscosity increase at 24° C. from 10.5 to 13 seconds(Ford-cup, 4 mm nozzle).

198 g of bulk polymerizate, 2 g of divinyl benzene, about 50%, 7 g of2,2'azobis(2,4-dimethylvaleronitrile) were emulsified with 828 g of 3g/kg sodium dodecylsulphate in an Ultra Turrax for some minute.

The pre-emulsion was charged to a 2-step Manton Gaulin homogenizer,model 15 M, where a narrow drop size distribution of 0,19 μm (determinedwith a Coulter Nanosizer) was obtained. The homogenized emulsion and 1 gof sodiumdodecylsulphate were charged to a 1,5 l glass autoclave whichwas placed under nitrogen gas atmosphere. The emulsion was polymerizedat 65° C. during 12 hours. In this way a 0,19 μm, 19% microsuspensionwith charge modifier incorporated during polymerization and cross-linkedwith 0,5% divinyl benzene was obtained.

EXAMPLE 3

Preparation of base particle.

To a solution of 2 kg 0.16 molal trisodium phosphate 520 g of 1.0 molalcalcium chloride solution was added under agitation and finally 150 g ofa 0.2% solution of sodium dodecylbenzenesulphonate. The now obtainedmixture was diluted to 2965 g with 0.2% potassium dichromate solutionand forms the dispersion medium.

700 g of styrene, 300 g of buthyl methacrylate, 80 g of carbon black"Printex V" (Degussa) and 3 g of neozapon schwartz X 51 (BASF) weredispersed in a ball mill to give a carbon-monomer dispersion.

10 g of 2,2-azobis(2,4-dimethylvaleronitrile) was dissolved in 990 g ofcarbon-monomer dispersion and added together with the dispersion medium(2965 g) to a reactor. The mixture was put under nitrogen gas atmosphereand allowed to polymerize at a rapid heating from room temperature andfurther at 85° C. for 1 hour under moderate agitation. The mixture wascooled to room temperature and pH adjusted to about 3 whereby 35 g of2,2-azobis(2,4-dimethylvaleronitrile) was added. After agitation for acouple of minutes pH was adjusted to about 9 and the mixture wasrecirculated through a dispersion unit "Ystral" (Bergius Trading AB) andemulsified to a suitable drop size for toner particles. The reactor wasonce again placed under nitrogen gas atmosphere and polymerizationcontinued for 18 hours at 65° C. under moderate agitation. Thesuspension was then cooled to room temperature.

A part of the polymerizate was transferred to a vessel and pH adjustedto 2 with HCl and calcium phosphate which functions as protectivecolloid was hereby dissolved. The suspension was filtered and firstwashed with acidified water and then with distilled water to finally bedried at 35° C.

In this manner toner particles with a particle size of about 10 μm andwith a charge of -12 uC/g against a Hoganas carrier were obtained.Copying tests with the toner in a Mita DC 313 Z first gave good copyingresults but the reproducability was fairly rapidly impaired due to thestrong adhesion of the spherical, smooth toner particles to thephoto-drum.

EXAMPLE 4

Coating of base particles with fine-grained particles.

70 g of 10% polystyrene latex prepared according to Example 1, 390 g of1.5 g/kg sodium dodecylsulphate and 2 l of water was mixed and form thecoating dispersion.

2 kg 1.5 g/kg sodium dodecylsulphate were charged to a reactorcontaining 4 kg of a suspenion prepared according to Example 3 and pHwas then adjusted to 2 with HCl. The coating dispersion was then addedfor 20 minutes under good agitation and then mixed for totally 1 hourbefore the temperature was raised to 65° C. At 65° C. pH was adjustedwith NH₃ to 8.3 and the temperature further raised to 90° C. After lessthan 5 minutes at 90° C. the now coated toner suspension was cooled toroom temperature.

Subsequently pH was adjusted to 2 using HCL and calcium phosphate, whichfunctions as protective colloid, was hereby dissolved. The suspensionwas filtered and first washed with acidified water and then withdistilled water. A sample was doped with 0.05% neozapan schwartz X 51,based on the amount of polymer, by slurrying the filtercake in waterafter the washing and mixing with a 1% methanol solution of the chargemodifier and filtration was then carried out again.

Investigation with a scanning electron microscope showed that thepolystyrene particles had been adsorbed by the base particles and, dueto the heat treatment, been melted into the base particles to about halftheir volume. The amount of polystyrene latex used was so that about 10%of the surface of the base particles was covered by fine-grainedparticles. In this manner toner particles with a "pimply" surface wereobtained and they gave a charge of -14 uC/g against a Hoganas carrier.Copying tests with a Mita DC 313Z did from the beginning give very goodcopies and the reproducability was still good after 30000 copies. Aparticularly good background and clean copies were noted. The photo-drumwas only covered by small amounts of toner which could be removedeasily.

EXAMPLE 5

Coating of base particles with fine-grained particles.

Base particles were prepared in the same manner as in Example 3 andcoated in the same manner as in Example 4 but with 40 g of 19%polystyrene latex prepared in accordance with Example 2.

Microscopy showed that the particles had been adsorbed and melted intothe base particles to about half their volume and the degree of surfacecoating was also here in the order of 10%. In this manner pimply tonerparticles were obtained where the "pimples" contained a charge modifierwhich gave a charge of -17 uC/g. As the toner of Example 4 this toneralso had excellent copying properties.

EXAMPLE 6

The process of Example 4 was repeated with the difference that theamount of latex was lowered to 35 g.

Microscopy showed a lower degree of covering, about 5%. Despite thistoner particles obtained better copying properties than toner particlesaccording to Example 3.

EXAMPLE 7

The process of Example 4 was repeated with the differens that the amountof latex was lowered to 7 g. This covering corresponds about 1% of thesurface but despite the low covering a positive effect compared toexample 3 was noted.

EXAMPLE 8

Preparation of base particle suspension.

The process of example 3 was repeated with the difference that inconnection with the addition of 35 g of2,2-azobis-(2,4dimethylvaleronitrile) at pH 3, 8 g ofdimethylaminoethylmetacrylate were also added. In this way a suspensionof base particles was prepared with a Zeta-potential changing frompositive to negative with increasing pH at a higher pH value than forthe suspension in Example 3.

EXAMPLE 9

Preparation of a base particle suspension.

The process of Example 3 was repeated with the difference that afteremulsifying to suitable drop size for toner particles 11 g oftrimethylaminoethylmethacrylate bromide were added. In this way asuspension of base particles was prepared, with a Zeta-potentialchanging from positive to negative at a higher pH than for thesuspensions of Example 3 and 8.

EXAMPLE 10

Preparation of a fine-grained polymerizate.

1 g of 2-sulfoethylmethacrylate was added to an autoclave together with309 g of water and pH was adjusted to about 4 with NH₃, and 0,4 g of 1mM CuSO₄ solution and 80 g of styrene were added. The mixture was heatedto 80° C. At 80° C. 10 g of 3,5% H₂ O₂ was added at the same time as themixture was put under nitrogen gas atmosphere. The polymerization wasthen allowed to continue for 12 hours, which resulted in a 0,14 μm, 20%latex which was free from surfactants.

40,6 g of the above prepared latex, 200 g of water and 0,4 g of 1 mMCuSO₄ solution were added to an autoclave equipped with two droppingfunnels. To one dropping funnel 72 g of styrene was added and to theother dropping funnel 76 g of water and 1 g of 2-sulfoethylmethacrylate,for which pH was adjusted to about 4 (NH₃), were added. The autoclavewas heated to 80° C. At 80° C. 10 g of 3,5% H₂ O₂ was added at the sametime as the whole system was put under nitrogen gas atmosphere. Thecontent of both the dropping funnels were added during about 3 hours.The polymerization was then allowed to continue for 12 hours. In thismanner a surfactant free coating latex was prepared with particles of0,26 μm and a dry content of 20%.

EXAMPLE 11

Coating of base particles with latex.

26 g of 20% coating latex prepared according to Example 10, 90 g of 5g/kg sodiumdodecylsulphate and 1260 g of water were mixed and constitutethe coating dispersion. This amount of coating dispersion correspondsabout 5% surface covering of the base particles.

To an autoclave containing 4 kg suspension of base particles, preparedaccording to Example 8, 480 g of 5 g/kg sodiumsulphate and 3520 g waterwere added. The coating dispersion was added during 20 minutes at goodagitation, without preceding adjustment of pH and thus the protectivecolloid was not dissolved. The mixture was allowed to stand underagitation for an hour before the temperature was raised to 90° C. Aftera minute or so at 90° C. the coated toner suspension was cooled to roomtemperature. The suspension was acidified to pH 2 and filtrated andwashed with water. The sample was doped in the same manner as in Example4 and in this way pimply toner particles with good copying qualitieswere obtained.

The example illustrates coating of base particles with latex, when theprotective colloid was present. Thus it was possible to bring togetherlatex- and base particles and heat the mixture to obtain adhesionbetween base- and latex particles, in spite of the presence ofprotective colloid consisting of precipitated calcium phosphate.

In another test the method according to this test was repeated, with thedifference that the base particles were prepared according to Example 9.The test showed that the same result was obtained irrespective of thebase particles being prepared according to Example 8 or 9. When both thetests were repeated with the difference that the latex was preparedaccording to Example 1, the latex agglomerated in the water phase whenthe temperature was raised and thus no markedly pimply toner particleswere obtained. The same thing happened when the test was repeated withbase particles prepared according to Example 3 and latex preparedaccording to Example 1. Also when the test was repeated with baseparticles prepared according to Example 3 and latex prepared accordingto Example 10, no ramaining coating was obtained after the drying. Thusit was possible to coat base particles with latex also in the presenceof active protective colloid, when as well base particles as latexparticles were copolymerized with small amounts of functional monomeresbut of opposite charge character.

EXAMPLE 12

Preparation of anionic latex.

110 g of styrene, 0.33 g of sodiumdodecyl sulphate and water to a totalof 1067 g were charged to a 1.5 l glass reactor with a double jacketingand equipped with agitator and valve for evacuation and introduction ofnitrogen gas. The mixture was heated to 80° C. under rapid agitation. At80° C. 33 g of a 1% potassium persulphate solution was added and at thesame time the mixture was subjected to nitrogen gas atmosphere. Thepolymerization was allowed to continue for 12 hours and resulted in a0.28 μm 8% seed latex.

300 g of the above seed latex. 0.30 g of sodium dodecylsulphate andwater to a total of 1060 g were charged to the same equipment as above,but in this case a dropping funnel was also connected. 100 g monomer wascharged to the funnel. Depending on the desired composition of the finallatex varying amounts of methacrylic acid, divinyl benzene (about 50%)and styrene were charged. In this example 1.8 g of methacrylic acid, 24g of about 50% divinyl benzene and 74.2 g of styrene were charged andthis is presumed to give a polystyrene latex cross-linked with 9.7%divinyl benzene (100%) and which also contains 1.5% methacrylic acid.

The mixture in the glass reactor was heated to 80° C. under moderateagitation. At 80° C. 40 g of a 1% potassium persulphate solution wasadded and at the same time the mixtures in the reactor and in the funnelwere subjected to nitrogen gas atmosphere. The monomer mixture was thenallowed to drop drown into the reactor for about 3 hours. Thepolymerization was then allowed to continue for 12 hours which gave an0.48 μm 9% latex which is presumed to have the above stated composition.

EXAMPLE 13

Preparation of toner particles with pimply surface.

To a solution of 2 kg of 0,16 molal trisodium phosphate 520 g of 1.0molal calcium chloride solution was added under agitation and finally150 g of 0.2% sodium dodecylbenzene sulfonate was added. The nowobtained mixture was diluted to 2965 g with 0.2% potassium dichromatesolution and forms the dispersion medium.

700 g of styrene, 300 g of butyl methacrylate, 80 g of carbon black"Printex v" (Degussa), 3 g of charge modifier "Neozapon Schwartz X51"(BASF) and 3.5 g of azobisisobutyronitrile and 50 g of polyethylene wax,as release agent, were dispersed under controlled temperature in a ballmill. During the process the temperature was allowed to slowly reach105° C. 1 kg of fairly warm carbon-monomer dispersion was charged to anautoclave containing 100 g of 9% latex prepared according to Example 12and water to a total of 1 kg. During slow agitation HCl was added untilthe latex had been absorbed by the monomer phase. This was establishedby simple microscope investigation. The mixture was then made alkaline(pH about 9) by NH₃ and 35 g of 2,2azobis(2,4-dimethylvaleronitrile)were added. After agitation for a couple of minutes the dispersingmedium (2965 g as above) was added. The agitation in the reactor wasintensified so that satisfactory agitation was obtained and the mixturewas recirculated through a dispersing unit "Ystral" (Bergius Trading AB)and was hereby emulsified to a suitable size for toner particles. Thereactor was put under nitrogen gas atmosphere and polymerization wasallowed to continue for 18 hours at 65° C. under moderate agitation. Thesuspension was then cooled to room temperature, pH was adjusted to 2with HCl and calcium phosphate, which functions as protective colloid,was hereby dissolved. The suspension was filtered and washed first withacidified water and then with clean water and finally dried at 35° C.Investigation with scanning electron microscope showed that thecross-linked polystyrene latex particles with 1.5% methacrylic acid hadbeen oriented towards the phase boundary surface of the toner particlesso that about half the volume of the latex particles protruded out fromthe surface.

Copying tests in a Mita DC 313Z with a Hoganas carrier gave excellentcopying properties and the reproducability was still good after 30000copies.

EXAMPLE 14

This example shows how the position of the latex particles in the phaseboundary surface of the toner particles can be controlled at theproduction of pimply particles.

Four latexes with varying contents of methacrylic acid, 0.5%, 1%, 2% and2.6%, and a constant amount of divinyl benzene 9.7% were preparedaccording to Example 12.

In the same manner as in Example 13, 0,5% of the above described latexes(calculated as dry latex) were in turn added to 1 kg of the warmcarbon-monomer dispersion which was then emulsified and polymerized inaccordance with the procedure described in Example 13. In this mannerfour different types of toner with varying roughness were obtained.Investigations with scanning electron microscope showed that:

(1) Toner particles in the case wherein latex with 0.5% methacrylic acidhad been absorbed had an almost smooth surface.

(2) Toner particles with 1% methacrylic acid latex had an uneven surfacebut only a minor part of the diameter of the latex particles protrudedabove the phase boundary surface of the toner particles.

(3) Toner particles with latex containing 2% methacrylic acid had anuneven surface and the latex particles were well visible above the phaseboundary surface. It was estimated that a little more than half thediameter protruded out from the phase boundary surface.

(4) Toner particles with latex containing 2.6% methacrylic acid also hadwell visible latex particles in the surface but the latex was found tobe even more protrusive than before.

In all cases the unevenesses were fairly uniformly distributed over thesurface of the toner particles. Copying tests in a Mita DC 313Z withHoganas carrier gave excellent copying results above all with tonertypes with latex containing a higher part of methacrylic acid.

EXAMPLE 15

Preparation of amphoteric latex.

To the same equipment as in Example 12 1.5 g of "Querton 16Cl29"(KenoGard), 1.5 g of 1 mM CuSO₄ and water to a total of 1032 g wereadded. To the connected funnel 150 g of styrene charged. The mixture inthe glass reactor was heated to 80° C. under moderate agitation. At 80°C. 18 g of 3.5% H₂ O₂ added and at the same time the mixtures in thereactor and the dropping funnel were placed under nitrogen gasatmosphere. The monomer was added to the reactor during about half anhour. After additionally 2 hours 1.5 g of Querton 16Cl29 charged and thereactor again placed under nitrogen gas atmosphere. The polymerizationwas then allowed to continue for 10 hours which resulted in a 0.13 μm11% seed latex.

150 g of a seed latex, 3.3 g of Querton 16Cl29, 1.5 g of 1 mM CuSO₄ andwater to a total of 1082 g were charged to the same equipment as above.To the dropping funnel 103.5 g of monomer charged. Depending on thedesired composition of the final latex varying amounts of(2-dimethylaminoethyl)-methylacrylate (DMAEMA), methacrylic acid,divinyl benzene (about 50%) and styrene were charged.

In this Example 4.71 g of DMAEMA, 0.51 g methacrylic acid, 12 g of about50% divinyl benzene and 86.28 g of styrene were charged and presumed togive a polystyrene latex, cross-linked with 5% divinyl benzene (100%)and which also contains 3.9% DMAEMA and 0.4% methacrylic acid.

The pH of the mixture in the reactor was adjusted to 2 with HCl andheated to 80° C. under moderate agitation. At 80° C. 15 g of 3.5% H₂ O₂added and the mixtures in the reactor and the funnel were at the sametime subjected to nitrogen gas atmosphere. The monomer mixture was thenallowed to drop down into the reactor for about 3 hours. Thepolymerization was then allowed to continue for 12 hours and thisresulted in a 0.2 μm 9% latex which is presumed to have the above statedcomposition.

EXAMPLE 16

Preparation of toner particles with "pimply" surface using amphotericlatex.

In the same manner as in Example 13 0,3% of amphoteric latex (calculatedas dry) prepared according to example 15 were absorbed in the warmmonomer-carbon dispersion, but the absorption took place in alkalineenvironment with NH₃ /NaOH. After the absorption pH was adjusted toabout 9 and the mixture emulsified and polymerized in accordance withExample 16.

Investigation with a scanning electron microscope showed that the latexparticles of 0.2 μm had been oriented towards the surface.

In the same manner as in Example 15 additionally two latexes wereprepared which had the same dry contents calculated in moles of amineand carboxylic acid groups but the ratio was changed from 5:1 to 1:1 and1:5 respectively of amine and carboxylic acid. In the same manner asabove two additional types of toner particles were prepared using thesenew latexes.

Investigation in microscope showed that also for the two later types the0.2 μm latex particles had been oriented towards the surface. In thesecases the basic shape of the toner particles was still smooth but partof the diameter of the latex particles protruded out from the phaseboundary surface.

In this manner particles suitable for copying of electrostatic printingwere prepared.

Example 17-19 relate to the application of fine-grained particlesaccording to a dry method.

EXAMPLE 17

Preparation of a fine-grained polymerizate.

The process of Example 2 was repeated with the difference that divinylbenzene and sodium dodecylsulphate were excluded. The organic phase wasinstead emulsiefied with 828 g of 3 g/kg ammonium laurate. In this way a0,2 μm microsuspension containing charge modifier was obtained.

EXAMPLE 18

Coating of base particles with fine-grained particles in a dry process.

4 kg of suspension of base particles, prepared according to Example 3,were acidified, filtered and washed with water. The filter cake wasfinally dried at 35° C. The microsuspension, prepared according toExample 17, was precipitated by adding acid and airdried in thin layersspread on glass sheets at 30° C. 1 kg of dry base particles and 4 g ofdry fine-grained particles, obtained from the dried microsuspension,were charged together with polyethylen granules, about 4 mm in size, toa powder mixer and mixed for 1 hour. The temerature of the powder bedwas then raised during continued agitation to 55° C. and held there for30 minutes in order to adhere the fine-grained particles to the baseparticles. Finally the temperature was raised for a short period toabout 70° C. in order to partly melt the fine particles into the surfaceof the base particles, where upon the powder bed was cooled to roomtemperature. The coated base particles was then separated from thegranules by sieving.

In some cases the toner particles were mixed with aerosil R972 tosupport good powder and triboelectrical properties.

In this way toner particles with good copying properties were obtained.

EXAMPLE 19

Coating of base particles with fine-grained particles.

4 kg of suspension of base particles, prepared according to Example 3were acidified, filtered and washed with water. The filter cake wasfinally dried at 35° C. 1 kg of dry base particles was charged togetherwith polyethylen granules about 4 mm in size to a powder mixer. Vacuumwas applied to the mixer and the jacket temperature was adjusted to 30°C. The microsuspension from Example 17 was added on portions of 2 ml.About 5 ml/min. were added and the addition of dispersion was stoppedafter 20 ml, which was equal to 4 g of dry fine-grained particles. Themixer was operated until the mixture became dry, whereupon the vacuumpumping was interrupted and the temperature was increased duringcontinued agitation to 55° C. and after that to 70° C. in the samemanner as in Example 18.

The particles were then prepared in the same manner as in Example 18.

In this way toner particles with good copying characteristics wereobtained.

We claim:
 1. Toner for electrophotographic copying or electrostaticprinting, characterized in that it comprises an internally pigmentedthermoplastic base or main particle prepared by suspensionpolymerization and having a mean diameter within the range of from 2 to25 μm, the surface of the base particle being covered by a fine-grainedpolymerizate prepared by emulsion- or microsuspension polymerization andhaving a mean diameter of from 0.05 to 33 percent of the mean diameterof the base particle and whereby from 1 up to 10 percent of the surfaceof the base particle is covered by the fine-grained polymerizate. 2.Toner according to claim 1, characterized in that the mean diameter ofthe fine-grained polymerizate is from 0.2 to 15 percent of the meandiameter of the base particle.
 3. Toner according to claim 1,characterized in that also the fine-grained polymerizate is internallypigmented.
 4. Toner according to claim 1, characterized in that themelting point of the fine-grained particles is >10° C. higher than thatof the base particles.
 5. Toner according to claim 1, characterized inthat the fine-grained particles have been prepared by microsuspensionpolymerization and contain a charge modifying agent.
 6. A method for thepreparation of a toner for electrophotographic copying or electrostaticprinting according to claim 1, characterized in that an aqueousdispersion of an internally pigmented thermoplastic base particleprepared by suspension polymerization and having a mean diameter withinthe range of from 2 to 25 μm is brought into contact with a latex of afine-grained polymerizate having a mean diameter of from 0.05 to 33percent of the mean diameter of the base particle whereafter thetemperature is raised so that the fine-grained particles adhere to thesurface of the base particles to a degree of covering of from 1 up to10%.
 7. A method according to claim 6, characterized in that the aqueousdispersion of the pigmented base particle contains polymerization serumfrom the polymerization of the base particle, which serum contains aprotective colloid system.
 8. A method according to claim 6,characterized in that an aqueous dispersion of the pigmented baseparticle is brought into contact with a latex of the fine-grainedpolymerizate whereafter a protective colloid system is formed in theaqueous dispersion and the temperature is raised so that thefine-grained particles adhere to the surface of the base particles.
 9. Amethod according to claim 7, characterized in that the protectivecolloid system is deactivated before the adding of the latex of thefine-grained polymerizate, whereafter the protective colloid system isreformed and the temperature is raised.
 10. A method according to claim7, characterized in that latex of the fine-grained polymerizate havingfunctional groups of opposite charge than the base particles is addedwhereby the latex particles are attracted to the base particles andthereby penetrating the colloid layer.
 11. A method for the preparationof a toner for electrophotographic copying or electrostatic printingaccording to claim 1, characterized in that an internally pigmentedtermoplastic base particle having a mean diameter within the range offrom 2 to 25 μm is prepared by suspension polymerization of a monomer ora monomer mixture in the presence of a latex of a fine-grainedpolymerizate having a mean diameter of from 0,05 to 33 percent of themean diameter of the base particle, the particles of the latex beingsubstantial insoluble in the monomer and said latex particles having ahigher degree of hydrophilicity than the polymer material in the mainparticles formed at the suspension polymerization.
 12. A methodaccording to claim 11, characterized in that the latex polymer materialoriginates from a monomer mixture which for control of hydrophilicitycontains a monomer with anionic character in alkaline environment.
 13. Amethod according to claim 11, characterized in that the latex polymermaterial originates from a monomer mixture which for control ofhydrophilicity contains a monomer with cationic character in acidenvironment.
 14. A method according to claim 11, characterized in thatthe latex polymer material originates from a monomer mixture which forcontrol of hydrophilicity contains a monomer with an amino group.
 15. Amethod according to claim 11, characterized in that the latex polymermaterial originates from a monomer mixture which for control ofhydrophilicity contains a monomer with hydroxyl group.
 16. A methodaccording to claim 11, characterized in that the latex particles arecross-linked.
 17. A method for the preparation of a toner forelectrophotographic copying or electrostatic printing according to claim1, characterized in that a dry internally pigmented thermoplastic baseparticle prepared by suspension polymerization and having a meandiameter within the range of from 2 to 25 μm, is mixed with fine-grainedplastic particles having a mean diameter of from 0.05 to 33 percent ofthe mean diameter of the base particles in an amount sufficient to coverfrom 1 up to 10% percent of the surface of the base particle, whereafterthe temperature is raised so that the fine-grained particles adhere tothe surface, whereby the fine-grained particles have a higher meltingpoint than the base particles.